Seeming and one other spell
To cover the illusions on both the creatures and the treasure you will need at least two spells. One to cover the creatures and the other for the treasure.
Seeming for the creatures
The most important spell you need is seeming. It will let you disguise the farmer and all his pigs as a 5th level spell. From its description:
This spell allows you to change the appearance of any number of creatures that you can see within range. You give each target you choose a new, illusory appearance. An unwilling target can make a Charisma saving throw, and if it succeeds, it is unaffected by this spell.
The spell disguises physical appearance as well as clothing, armor, weapons, and equipment. You can make each creature seem 1 foot shorter or taller and appear thin, fat, or in between. You can't change a target's body type, so you must choose a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you. The spell lasts for the duration, unless you use your action to dismiss it sooner.
Hopefully the pigs are willing so the saving through won't happen, otherwise you might have some pigs among your hellhounds. Otherwise the most important parts of this spell are the 8 hour duration and lack of concentration. This lets the farmer set this up and still have concentration available for the second illusion.
Second spell for treasure
There are a number of options for second spell. Illusions of inanimate objects like treasure can even be handled with a cantrip (minor illusion). Spell such as; silent image, major image, creation, and programmed illusion are all decent options. The higher level options have the advantage of longer durations and no concentration. Programmed illusion is definitely the most convincing and only needs to be cast once as it lasts until dispelled, but is a 6th level spell.
You didn't explicitly ask for this but I thought it may be a good idea. Hallucinatory terrain will allow you to make the cave look like a devils lair instead of a hole in the ground. I have used it to great effect NPC villains in my own campaign. It has the additional benefit of allowing you to hide pit traps and other dangers from the party, further disguising the true nature of what is going on here.
Alternatively you could use mirage arcane to get the benefits of both this and the treasure illusion with a single 7th level spell. That has the added bonus of standing up to physical inspection.